Typical geological characteristics and exploration practices of lacustrine shale oil: A case study of the Kong-2 member strata of the Cangdong Sag in the Bohai Bay Basin

Successful development of marine shale oil in North America has advanced the researches on lacustrine shale oil in China. However, there remains no unified understanding of certain issues, such as the exploration prospects of lacustrine shale oil. To determine the typical geological characteristics and exploration potential of lacustrine shale oil in China, the shale strata of the Kong-2 Member in the Cangdong sag was selected as a study system. The geologic setting, characteristics, and distribution of sweet spots, as well as the exploration prospects of lacustrine shale oil were analyzed using 635.8 m cores and the analytical data for 12,000 core samples from three wells,G108-8, GD12, and GD14; oil testing and production testing data from multiple wells were also evaluated. Results show that the shale oil developed in a slightly reductive and tectonically stable sedimentary environment. The shale strata are composed of various minerals, including mainly quartz, feldspar, and dolomite, with abundant laminae. Samples with total organic carbon (TOC) values > 1% account for>75% of the shales, which are primarily type I and type II-1 kerogen with thermal maturities between 0.5% and 1.1%, and formed during the oil window. These lacustrine shales have multiple types of reservoir spaces, including organic pores, intercrystalline pores, intergranular pores, microfractures, and laminar fractures, with an average (all averages are arithmetic means) OSI index (pyrolysis S1 × 100/TOC)of 147.1 mg oil/g TOC, and an average brittleness index >50. The reexamination of old wells and horizontal well exploration aimed at the sweet spots of type Ia and Ib oils have achieved promising results. Well KN9 had a daily oil production of 29.6 m3 after fracturing, and horizontal well GD1701H exhibited its highest daily oil production at 75.9 m3, and currently produces 21–24m3consistently. Well GD1702H had a maximum daily oil production of 61 m3, and stably produces 20–25 m3 at present. The resources of the Ek21SQ⑨ sequence calculated using amass oil content method were 208 million tons, confirming the extensive exploration potential of shale oil in this lacustrine lake basin.